<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <script src="https://cdn.bootcss.com/vue/2.5.17/vue.js"></script>
</head>

<body>
    <div class="app">{{msg}}</div>
</body>
<script>
    const app = new Vue({
        el: document.getElementsByClassName("app")[0], // .app
        data: {
            msg: "hello world"
        }
    })
    console.log(app.$el)
    console.log(app.msg) //hello world
</script>

</html>